Conventional electrophotographic printers are constructed with an image fixing station spaced downstream from an image transfer station. Print media, such as paper, is moved through the image transfer station and then through the image fixing station. At the image transfer station, an electrophotographic image is transferred to the print media. At the image fixing station, the image is affixed to the print media. The print media is moved through the stations by a media motion assembly which typically includes a roller assembly located at the image fixing station, a guidance system positioned intermediate of the image transfer station and the image fixing station, and a motor connected to drive the roller assembly and the guidance system.
To form an electrophotographic image on a print media, an image forming device at the image transfer station places multiple scan lines at a preset scan rate on a cylindrical photoconductive drum. A series of scan lines forms an electrostatic image on the drum. The drum is then rotated past a toner applicator which applies a toner or ink powder to the electrostatic image. The drum is next rotated adjacent the print media to transfer the toner image to the print media, which is moving through the image transfer station and past the drum at a constant velocity. Thereafter, the media is moved through the image fixing station wherein the roller-fusing assembly fuses the toner image onto the media.
The roller-fusing assembly includes a fuser roller and a pressure roller. The media is passed between the two rollers and moved by the rotation of the two rollers. The fuser roller is heated to a predetermined temperature conducive to fusing the toner image onto the media.
The velocity of the media past the image transfer station must be constant; otherwise, image quality is severely diminished. The media velocity is independent of the scan rate of the image forming device. For optimum printing quality, the media velocity and the scan rate should be synchronized. For example, in one electrophotographic printer, the optimum printing synchronization may be 300 scan lines per inch of print media. Accordingly, the optimum media velocity is that which permits the drum to transfer 300 scan lines per inch to the print media as the media moves past the image transfer station.
Although a velocity allowing the optimum number of scan lines per inch is ideal, many factors influence the printer to alter slightly the velocity of the media. One of the factors is fuser temperature in the fuser roller. As the fuser temperature changes, the fuser roller expands and contracts about its radial axis. This change in radial dimensions affects the velocity at which the media moves between the fuser roller and the pressure roller.
Prior art electrophotographic printers make no adjustment to media velocity for fluctuations in the fuser temperature.
An advantage of the present invention is to provide an electrophotographic printer which adjusts the media velocity for fluctuations in the fuser temperature. In this manner, the optimum media velocity is maintained throughout the printing process.
These and other advantages of this invention will become apparent upon reading the following detailed description of a preferred embodiment.